The disclosed embodiments of the present invention relate to satellite positioning, and more particularly, to a satellite-based positioning method and an associated apparatus.
A Global Navigation Satellite System (GNSS) receiver determines its position by computing arrival times of signals transmitted simultaneously from multiple satellites. Current fully-operational GNSS include the US GPS (Global Positioning System) and the Russian GLONASS (GLObalnaja NAvigatsionnaja Sputnikovaja Sistema). As part of their messages, these satellites transmit both satellite positioning data and satellite clock timing data. The satellite positions and clock timing are represented by almanac or ephemeris data, wherein the ephemeris data provides an extremely accurate estimate (˜1 meter error) of satellite positions and clock bias. The process of searching for and acquiring satellite signals, reading the ephemeris data transmitted by the satellites, and computing the location of the receiver from this data is time consuming, however, often requiring several minutes. In many cases, this lengthy processing time is unacceptable and furthermore, greatly limits battery life in miniaturized portable applications.
For example, a GNSS receiver determines position based on the measurement of the arrival times at a GNSS receiver antenna of the GNSS signals broadcast from orbiting satellites. As stated, one disadvantage of such a system is the relatively long time needed to perform signal acquisition under certain conditions. Satellite signals cannot be tracked until they have first been located by searching in a two-dimensional search “space”, whose dimensions are code-phase delay and observed Doppler frequency shift. The process of the GNSS receiver searching for, acquiring, and demodulating satellite signals is sometimes referred to as a “standalone” mode of operation, which can be contrasted with an “assisted” mode of operation.
In order to reduce the delay associated with a stand-alone mode of operation, information may be provided to aid a GNSS receiver in acquiring a particular signal. Such assistance information permits a receiver to narrow the search space that must be searched in order to locate a signal, by providing bounds on the code and frequency dimensions. A system that employs a GPS receiver augmented with externally sourced GPS assistance data is commonly referred to as an “assisted global positioning system” (AGPS).
AGPS is a system that predicts satellite orbit information and, in many cases, greatly improves the startup performance or time-to-first-fix (TTFF) of a GNSS satellite-based positioning system. Extended Prediction Orbit (EPO) is a prior art example of an AGPS with off-line server based AGPS technology, which is a swift way to obtain specific information of the satellite, such as position and clock, before the receiving of broadcast ephemeris. The specific information aids the GNSS positioning engine (PE) to quickly locate the satellite, and thus obtain a First Fix with a faster speed. Refer to US Patent No. 20110273329 for more details.
EPO supports up to 30 days satellite trajectory prediction using numerical fitting analysis with some models (mathematical, force, etc.), and greatly enhances a user experience by improving TTFF of a GNSS receiver. While providing a prediction of the extended satellite orbit and clock up to many days ahead, however, errors between the predicted and the actual orbit with respect to some satellites may be enormous. The First Fix may be poor since the GNSS PE cannot foretell quality of the orbit and clock prediction for respective satellites in advance.
Therefore, a novel satellite positioning method is desired for aiding a GNSS receiver to derive a First Fix.